In Long Term Evolution (LTE) systems, a random access procedure is used to achieve uplink synchronization for a terminal device, also known as User equipment (UE), which either has not yet acquired, or has lost, its uplink synchronization. There are two types of random access procedures, contention-based and contention-free.
FIG. 1 shows a signaling sequence of a contention-based random access procedure. As shown, the contention-based random access procedure involves four messages, referred to as MSG1, MSG2, MSG3 and MSG4, respectively. MSG1 and MSG3 are uplink messages from a UE to a network node (e.g., an evolved NodeB (eNB)) and MSG2 and MSG4 are downlink messages from the network node to the UE. The contention-free random access procedure involves only two messages, MSG1 and MSG2.
Both MSG2 and MSG4 are carried in Physical Downlink Shared Channel (PDSCH) and scheduled via Physical Downlink Control Channel (PDCCH). The scheduling information for MSG2 is transmitted in a common search space and the scheduling information for MSG4 is transmitted in a UE-specific search space. Since the PDCCH needs to be decoded based on Cell Reference Signal (CRS), the random access procedure in LTE depends on the CRS.
The next generation networks are expected to be highly energy efficient. One of the design principles for the next generation networks is to minimize static, always-on signals in order to reduce energy consumption at the network side and to reduce interference to UEs served by neighboring eNBs. As a result, the CRS may not exist in the future. Hence, it will perhaps not be possible for the UE to receive MSG2 and/or MSG4 based on the CRS any more.
There is thus a need for a solution for random access to enable the UE to receive MSG2 and/or MSG4 when no CRS is available.